A variety of modem technologies rely upon the precise deposition of a liquid material upon a solid substrate. Although it is frequently desirable to obtain a precisely uniform coating, methods for achieving such uniformity are often elusive.
A particular example is the semiconductor industry, in which resist materials (often photoresist materials) are deposited upon semiconductor wafers. The resist materials are subsequently exposed to light through a reticle and then portions of the resist are removed so that underlying portions of the substrate may be exposed to, for example, etchant or dopant species.
Conventional methods for applying photoresist involve the placement of a semiconductor wafer upon a chuck in a chamber. An arm having a dispensing tube at its end is positioned generally over the center of the wafer. A pump connected to the arm is activated and a puddle of photoresist is dumped onto the center of the wafer while it spins at a slow speed. Then the wafer is spun at a considerably faster speed. Centrifugal force tends to force the photoresist to move outward toward the edge of the wafer while surface tension and viscosity effects control the flow over the surface and the wafer edge boundary conditions. Eventually the entire wafer is coated with photoresist.
The conventional approach presents a variety of problems. First, a considerable amount of photoresist material is wasted. It is necessary to dump more photoresist onto the center of the wafer than is ultimately desired upon the surface of the finished wafer. Considerable excess photoresist is spun off the sides during the "fast spin" cycle of the process. Furthermore, the process typically produces an uneven photoresist coating, not unlike that depicted in FIG. 1. In FIG. 1, reference numeral 11 denotes a semiconductor substrate; while reference numeral 13 denotes a deposited photoresist. In a photoresist having a nominal thickness of approximately 1 .mu.m, the difference between the height of point 17 in the center of the wafer and point 15, at a location intermediate the center and edge of the wafers, may be 100.ANG. to 200.ANG.. Unevenness in photoresist layers is undesirable because it will induce variations in the size of critical features (normally placed in the same relative stack location). These variations are due to changes in the coupling of exposure light energy into the photo-active compound (PAC) in the resist. The variation in coupling is caused by the thin film standing wave effects between the top and bottom surfaces of the resist.
Furthermore, the equipment provided by many modem semiconductor manufacturers does not accurately position the dispense stream center over the center of the wafer. The resulting photoresist pattern is sometimes somewhat more complicated than that depicted by FIG. 1 because the initial deposit of photoresist is not in the center of the wafer. However, the resulting pattern has equally undesirable irregularities.
Those concerned with the development of the semiconductor arts as well as other fields in which the formation of coatings of precise thickness is desired have continued to search for methods to alleviate the above-described problems.